Frequency-equalization carrier system



Nov. l2, 1929. E, LGREEN 1,735,044

FREQUENCY EQUALZATION CARIER SYSTEM Filed oct; 2.2, 1927 3 sheets-sheetl W/f//m Eel/@#7 ,i J2e/n #J I Z/wfa mt. [IVM ATTORNEY Nv. 12, 1929. A fE. l. GREEN 1,735,044

' FREQUENCYl EQUALIZA'TION CARRIER SYSTEM Filed oct. 22, 1927 I- ssheets-(sheet 2 INVENTOR.

Il @feel/0 BY EM ATTORNEY Nov. 12, 1929. E GREEN l 1,735,044

FREQUENCY EQUALIZA'TYION CARRIER SYSTEM M1001. l M225. l

INVENTOR.

BY T/f i @L ATTORNEY rammed, Neef. i2, risas trasera Mdm@ Slita'ajfnsrematar jerarca f ESTILL I. GREEN, OF AS'GRANGE, NEW JERSEY, ASSIGNOR T0AMERICAN TELEPHONE AND TELEGRAPH COMFANY. CORPQBATION OFNEW YORK-rnnaunncy-naoamzarron chaman SYSTEM carri-er systems involvingtwo-Waytransmission to arrange all of the channels which transmit in onedirection Within one group or range of frequencies, the channelstransmittingin the opposite direction being arranged in `another groupor range of frequencies.

- 0bviously, in such a system, since the attenua-` tion is greater atthehigher frequencies, not

only willthe channel having the highest fremitting Yquency ofa group bemore attenuated-l than the channel ofthe same group having the lowestfrequency, but the'dil'erence in at-A tenuation willvbe'even more markedinthe case :of thechannel of" highest frequency transmitting in onedirection as'compared with the channel of lowest frequency transin the'opposite direction. Inaddition to the diderence in line attenuationfor` the dierent channels, there .will be differences in otherfactors,sueh vas noise, crosstalk, attenuation variation, gain,apparatus distortiom'etc., since these factorsdepend to a considerableextent upon the `frequency or frequencies used fora channel.

In order to overcome these diiliculties the invention contemplatesrearranging or trans- Iposing the order of the channels in .thefrequency spectrum from time to time as they are transmitted alongthetransmission line, this process being applied tov all channels regard,Lless of direction of transmissionjin such a way thatnot. only Awill anychannel be transmitted at the same average frequency levelas any otherchannel transmitting in thesame direction, but also at the samefrequency level as any channel transmitting in the opposite direction;yPreferably, the method is so carried out that the frequencies in eachchannel at the same 'average frequency level.

Application led Orztober4 22, A1927. Serial No. 228,021.

The fundamental advantage of such frequency equalization resides in thefact that it provides equalization with' respect'to all the featureswhich(areazfunction of frequency. The more important of these'factorsare line attenuation, noise, inter-system or inter-channelcross-talk, attnination variations, gain, apparatus distortion,'etc. Itshould be noted that the equalization provided in this manner i is muchsuperior to that ordinarily accomplished'fby particular means designedto take care of each oneof the above factors individually. Thisis'because frequency-equalization makes the condition'on all of thechannels.

the same as -theaverage condition, Whereas the other means ofequalization commonly used degradel all channels tothe condition of theWorst channel.

' The invent1on may now der-stoodl from the 'following description bemore fully uny when read in connection with the faccom' Verted andtransposed in accordance with one embodiment of the invention; Figs. 3lland 4 are schematic diagrams of repeaters.

arranged to invert the channels, frequency bands, and groups inaccordance with the l said layout in Fig. 2.; Fig. 5 is a lsymboliclayout in 'which the functions of group, channel,- and band inversionare carried out simultaneously; Fig. 6 is a schematic diagram of arepeater employed in connection with the system in Fig.. 5 forperforming the required inverting functions; Fig. 7 is a layout of thesystem as applied to a single channel two-way telephone system; Fig. 8is a simplified diagrain of a modulating repeater to be used in`connection with the system in Fig.- 7; Fig. 9v

showsfthe invention applied to amultiplex carrierY telegraph system;andFig. l0 is a symbolic diagram of the repeater to be employed in thesystem of Fig. 91

V@Figl 1A illustrates the present standard l .band a/e transposedlsoas-to be transmitted method of operation with -respect to frevthesections. Fer convenience, a system having six bands,.or three two-waychannels, is assumdialthough it will be understood that as in theotherflayout diagrams, each channel is represented by astraightmorizontal'line -havinga rectangular box near the middle thereofto which a legend is applied to indicate the character 'of the channel.Repeater points. are indicated by long vertical4 rectangles. VThedirection in which vthel'channel .transmits in each line section isindicated by the arrows attached to the'rectanglescontaining'the legendsofeach channel.

With respect' to the legends, it should'be noted that the letter W ofeach symbol indicates that the channel to which' it is appliedtransmits'ffrom eastv to west, while the letter vE appliedto a channellindicates that it transmits 'from'west to east The-subscript numeralsapplied to the letters W andE o'f the zg'ends have no relation to thevfrequencies assigned tothe channels, but merely indicate how thechannels are paired to form a plu-'-y rality \of vtwo-way communicationcircuits. Thus', the channel E1 is pairedwith the op- 'positelytransmittin channel W1, E2 *with W2, etc.l 'This num ering-of thebandsis immaterial, as in practice the bands may be paired in any mannerto give the best'results I regardless of whether they are equalized orza'tion.

not. `'Ihe frequency assignmentlor allocation of the various channels orbands is indicated in the diagram of Fig. lV and .other similar diagramsbythe vertical position of the, channel in the d iagram, the higherpositions representing higher frequencies. .The subscript N, associatedwith the symbols involvin the letters W and E in the diagram f Fig. 1,merely indicates that the variousl frequencies within the band of agivenchannel so desig- Iiated, are in their normal order and have not beeninverted. When the subscript I is used instead of N, as indicatedinFigs. 2, 5, 7, and

9, itmeaiis that the" frequency order Vofthe individual frequencies inparticular'bands so designated has been inverted.

A considerationof Fig. l shows that in the `arrangement thereillustrated the frequency position of eachband is unchanged throughoutthe system, and that none of the bands have been inverted. With such; anarrangement there is ne Fig. 2 is a-layout illustrating.one possiblearrangement for providing frequency equalization'. After a discussion ofthis arrangement, which will serve'to bring out the-principles involved,anda description of means for carry-ing it out in, practice, variousalternative arrangements will be considered.

. consideration of Fig. 2,which illustrates attempt at frequencyequalithe principles of the invention applied to a carriersysteminvol'vin four line-sections, will render it apparent t atduringtransmission4 over the lirst line section, which is the vsectionfrom the west terminal vto repeater the invention is applicable to anynumber of,v .channels In the diagram of Fig. l', as well No. l, thefrequency arrangement is identical with that -of the system of Fig. land all. the bands .are normal and not inverted. Upon-arriving atrepeater No'. 1,'however, the bands of,v thel upper group are 'invertedin `position With respect to each other` and the frequencies of eachlbandare thus inverted -with respect to their normal position, whileDuring transmission over. the second sec-' tion'of the linev (betweenrepeater No. 1v and"` repeater N o. 2) the channels are vtransmitted intheir rearranged frequency allocation with the-bands inverted but thedirectional groups of channels remain in their original fre-1 quencyrange. On arriving at repeater No. 2, however7 the bands of the lowervgroup aretransposed to the frequency assignment .of the upper groupandvice versa, without any change in the relative location of the bandsinthe group, and without reinversion of the frequencies within thebands. This is indicated by the dottedv lines at repeater No.2. Thisrepeater may, therefore, be vcalled a group inverter.

, During transmission over the third section of the line,l the channelsare transmittedas rearranged'by repeater No. 2. Upon arriving atrepeaterNo. 3'the channels in each group are reinverted, andalso the frequenciesofthe individual bands are reinverted.

The groups, however, are not re-transposed.

It will be apparent, therefore, that repeater No. 3 performs the sainefunctions as repeatioo er No. l, so that it is another channel' and efour line. stations the east terminal would obviously-be replaced by anordinary'repeatl er, and group inverters and channel and used atsucceeding band inverters would be repeaterpoints.

A little'l consideration will make it clear that in the system outlinedin Fig. 2 the avera'ge frequency of transmission is-the same for vall ofthe channels.' Thus, suppose the six frequency bands occupied by thethree It will be observed that the two-.way channels are 4-7, 8-'-11,12-15, -17-20,`21'24, and 2,5.-.28 kilocycles. The frequency allocationof'the different bands in the succesive sections'of the line of Fig. 2willl1 -then be' asindicated in -the"follow1ng ta e:

Average' Band- Sectionl Section2 Section?!l Sectionifwm,mcy

"1 circuits andl apparatus for these two types of repeaters areillustrated schematically in Figs. 3 and 4, where, for convenience', thefrequency assignments above assumed, are again'utilized. It will benoted that the channel and band inverter, shown in Fig. 8, consists oftwo parts or sides, one being utilized .for transmission in onedirection andv Athe otherfor transmissionin the opposite direction. Eachof thesefsides operates upon the double-modulation principle to producethe desired inversion of the order of the channels and inversion of thebands.

Each one-way path of the repeater at Fig. 3 includes two modulators,three filters and,

where necessary, an amplifier. ln the upper,

path, forexample, the lower group of channels passthrough the filterFEa'to'the modulator ME., where they heatv with 'a' carrier of 5 0kilocycles from a source Sa. |The filter FEb then functions to slect thelower side band resulting from the modulating action, thus resulting ininverting the order, of bands in the group, inverting the vorder offrequencies inthe band, and shifting the group of the` channels to ahigher range in the frequency spectrum (46 to 35 kilocycles). Theselected frequencies are then impressed upon the modulator ME, to beatwith the frequency of 31 kilocycles from the-source SEb. The filter FEl, then selects the lower sideband (15 to t kilocycles)l so that we nowhave the individual bands inverted and their order in the frequencyspectrum inverted, but the group as a whole lies in the same frequencyrange as it occupied before entering the repeater. It should be notedthat in this second modulating operation the selection of the lowerorder of the bands or the order ofthe frequen'cies within a given band,because the t and iS somewhat simpler than the arrange- -1 -band hasbeen inverted.v

carrier frequencylwas lower than the frequencies of the various bands.The amplifier AEc simply serves to amplify the selected bands to any.desired level. The apparatus in of the' repeater is similar the lowerone-way vpath to that in the f other path, the principal differencebeingthat the filters have different selective ranges and the secondmodulatorfMWb employs a different carrier source from the correspondingmodulator MEb of the...other path` Herev again the modulator MWa,together with the filter FVVminvertsy the orderof the Vbands and alsoinverts the bands themselves,

at the same time shifting the group of bands to Aa position somewhathigher inthe frequency spectrum. The shifted bands then beat with thefrequency of 5 kilocycles from the source SW1, in the modulator MWh, andafter the selection'of,l the lower ,sideband by Iilter FTWc wehave thesame channels as -be- 'fore'in the same frequency range as before, sofar as the group :is concernedbutthe order of the channels has beeninverted, and each In determining the carrier frequencies to be appliedto the first and second modulator ineachpatlnthe Jfrequencies are so,chosen that the difference between the two frequencies applied to thetwo modulators of the'path will be equal to the sum of the highest andlowest frequencies of the several bands to be modulated. For example, inthe case of the upper path, the bands range from 4 kilocycles to 15kilocycles. The sum of the upper and lower frequencies of the group istherefore 19 4kilocycles. The carrier frequency applied to the firstmodulator is'50 kilocvcles, and that to the second modulator is 31kilocycles, the difference between them being' 19 kilocycles. Similarly,inthe case of the lower path, the upper and lower frequencies of thegroup of bands are 28 kilocycles and 17 kilocycles, respectively, sothat their sum is kilocycles. The first modulator uses the same carrierfrequency of kilocycles as in the upper path. Therefore. the carrierfrequency applied to the second modulator must be 5 kilocycle's, so thatthe difference between the two carriers will be 45.1kilocycles.

It would be thoretically possible to use a single-modulation methodinstead of the double-modulation method which is illustrated in Fig. 3.but the use of the latter facilitates the elimination of the unwantedproducts of modulation bv making these fall in an unused part of thefrequency spectrum. lt may be noted in this connection that in anydoublemodulation scheme the frequency of the modulating carriers is notfixed but may be chosen soA as to minimize undersirable modulation orother effects.

The group inverter t'o be employed at repeater No. 2 of Fig. 2 isillustrated in Fig.

los

spectrum but shiftedto a higher position.

This upper sideband( group of bands) 'isselected by the filter FEc andthen. passed through. the amplifier AEc to the next line section. 4Thearrangement of apparatus in connection with the lower path of therepeater,

isexactlyjlike that of the upper path except that-f'transmission is inthe reverse direction. In connection with this arrangement, itwillbeobvious that the double-modulation method might also be used instead ofthe single-modulation method.

.In carrying out the arrangement of Fig. Q,channel andband inverters ofthe type illustrated in Fig. 3 would be used for repeaters N os. 1 and3. The direction of transmission' for the two, however, would bereversed, that ifs/Ato say, at repeater No. l'the left side oftheinverter 2 would face west, while at repeater N o. 2: 'it would faceeast. The terminal apparatus required in the system is evidently thesame as for normal operation, and therefore no description is necessary.

` yIt will be clear that the arrangement shown in Fig. 2 providesperfect frequency equalization for a system having four. line sections,and that by repetition of this .process it could be used' for'any systemhaving a total number of line sections evenly divisible by 4. Since itmay-be impractical to lay out systems with line sections in multiplesvof 4in many cases',

a system accomplishinglthe same resultswith two line sections would havedecided advantages. .The frequency arrangement of such a system isillustrated in Fig. 5, while the type of repeater -required'for suchasystem is,

This particular system, as illistrated'in Fig. 5, is shown 'applied to aline circuit having four-sections, and this involves-.the use, atrepeaters Nos. 1 and 3, of a type of repeater which performs thefunctions of inverting the frequencies within -each band, in: vertingtheorder of thebands in each group,

and transposing the groups. Tfhe resultantfrequency shifts are indicatedby the dotted 1 lines in repeatersNo.. 1 and No. 3 of Fig. 5.-

' transposed at repeater No. 1, so that the eastlIt will-be sengtliatthe channels are transmitted in their normal relation over thevfirst section of the line and are then completely bound -groupfchannels occupies the range formerly occupied by the westbound channels,and vice versa. It'will also be noted that the order of the channels inthe group is inverted. The channels are transmitted in their transposedrelation from the second and third line sections and then restored tonormal at repeater No. 3 for transmission over the fourth line section.

Repeater N o. 2 may obviously be an ordinary type of repeater. The typeof repeater used at repeaters No. 1 and No. 3, as already stated, is ofthe so-called group, channel and band inverter type. This type ofrepeater is illustrated in Fig. 6.` It'involves the same elements as thegroup inverter illustrated in Fig. 4r, with the essential difference,however, tha-t the carrier frequencyl supplied to the modulator ischosen upon a different principle. This frequency must be madesubstantially equal to the sum of thelowest frequency of the lowest handof the lower group and the yhighest frequency of the highest band of theupper group. In the case assumed, this frequency should'be in the"neighborhood `of 32 kilocycles. In the case of Fig. 4 the carrierfrequency is made equal to the difference between the two correspondingfrequencies .in

the two groups, for example, the difference between the. upper frequencyof the lower group and the upper frequency of the upper "group- In Ithearrangement of Figs. 5 and `(i. perfect frequency equalization isaccomplished in each set of two line sections, two being of Acourse theminimum'number of sections on -which this type -of equalization may be'obtained. The following table gives the frequency allocation of thebands in this system, on the assumption that the same band frequenciesare used as before.

' Average Band Section] Sect1on2 Section?, Sect1on4fmquen0y Thearrangements of Figs. 5 and 6 arerclatively Vsimple and represent thepreferred" method -of complete frequency equalization in a multi-channelcarrier telephone system.l

It might be thought of as a system in which .a number of frequencies aretransmitted'in such a manner that each occupies a certain position inthe available frequency range half of the time and occupies a conjugateposition the other half of the time. -While shown as applied to a*system of`four line sections, it'` can be used.`with a system having-only two line sections, but in this case a repeater of the type used atNo.Y 1 willbe employed between thetwo line sections, and a repeater ofthe type of No. Sat one of the terminals. For the purpose of transposingthe channels and then restoring the original relation ofthe I channelswhen the system is applied to'aline having av larger number of sectionsthan four, it may only be necessary to use two Special repeaters of thegroup', channel, and band inverter type, the repeaters, interconnectingthe other sections, being of theordinary type. Each of these specialrepeaters would be alike except that the bands of free transmission ofthe one repeater will be complementaryv to those of the other.The'alternativecut-oif frequencies for the two different locations areindicated in Fig. 6, the values for the one location bein indicated infull lines and the values for t c alternative location being indicatedin dotted lines.

It should be noted that'. the voice band may be included as one ofthe'c'liannel bands in tl'ie arrangement'I described. A simpleapplication of the frequency equalization principle the otherV rangefrom 4 to 7 kilocycles, is illusto a single channel system, of which oneband occupiesthe voice range, (0-3 kilocycles) and trated-in 7 and 8. Asbefore, the full, line cut-0E values for the filters in Fig. 8 are"those for 4the repeater when used in the posi tion ofrepeater No. l,while the-dotted figures will be the vcut-olf values of the filters whenthe repeater' is used at the position of repeater No. 3 of Fig. 7 Thelayout of the system, as p indicated in Fig. 7, will be obvious from thedescription of the previous arrangements, and need not be furtherdescribed. The system fillustrated in Figs. 7 and 8 might be used as asubstitute for the regular voice frequency circuit over which it wouldhave the advantage of avoiding the use of line balance at repeaterpoints.

The general principle of" the invention is I .applicable either tocarrier telephone or carrier telegraph systems. Figs.l 9 and l0 illustratea carrier telegraph systemar-ranged for frequency equalization. Thesystem is asf sumed to have eleven channels vwhich employ frequencies250 cycles apart in the two groups from 3.5 to 6.0,and 7.0 to 9.5kilocycles. The following table indicates the frequency posi i tion ofthe channels in the different sections:

^ ln Fig. 9 the upper-pair of lines, together f 60 with the associatedrectangle, represents a group of eleven channels transmitting in oneirection while the lower pair of linesand associated rectangle representthe corresponding eleven channels transmitting in the oppositedirection. As in the preceding iigures, the letters Wand E indicate thedirection of transmission. rlhe subscript llch indicates that there areeleven channels, and the subscript letter N or I, as the case may be,indicates whether the frequencies involved are in 'their normalpositions in the frequency spectrum or are inverted. The Ifrequencyinverter required is similar to those previously illustrated in Figs. 8and 6, the filter cut-offl frequency for repeater No. l bein indicatedin-full lines and those for repeater o. 3 being indicated in dottedlines in Fig. l0. The operation of the system will beobvious withoutfurther description. V l

It'willv be obviousthat the general princi- -ples herein disclosed maybe embodied in many .otherl organizations widely different from thoseillustrated, without departing from the spirit of the invention asdefined in the following claims. y

What is claimed is:

ity of frequency bands normally arranged in 'a givenorder in thefrequency spectrum for transmitting in .on'edrectiom a'correspon ingnumber of bands also normally arrangedy 1. In a'c'arrier systemcomprising a plural! in a given order for transmitting in the oppo- Isite direction, the method of equalization with respect to frequency,`which consists in transmitting the bands 4in both directions in normalrelation lover a portion of the transmission circuit,inverting theorderin the spectrum of the. bands transmitting in one direction, invertingthe order of the bands transmitting in the opposite direction, invertingthe order of the individual frequenT cies in each band, trarisposing thetwo sets of bands transmittingin opposite directions so that each setoccupies substantially the frequency space normally allotted vto theother, and transmitting the bands thus inverted and .transposed overanother portion of the transmission circuit. f

2. In acarrier system comprising a plurality of frequency bands normallyarranged 11i a given order in the frequency spectrum for L transmitting.in one direction, a corresponding numberl of bands also normallyarranged in a given order for transmitting in the opposite direction,the method of equalization With respect to frequency, which consists intransmitting the bands in both directions in normal relation over-aportion of the transmission circuit, inverting the order in the spectrumof the bands transmitting in one direction, inverting the order of thebands .A

transmitting in the opposite direction, transposing the two sets ofbands transmitting in i opposite directions so that each set occupiessubstantially the frequency space normally allotted to the other, andtransmitting 'the 'i bands thus .inverted and transposed over anotherportion of the transmission circuit.

3. In a carrier system comprising a plurality of frequency bandsnormally arranged in a given order in the frequency spectrum fortransmitting in one direction, a corre` spondi ranged vin agiven orderfor transmitting in 5 the opposite direction, the method of equalizationwith respect to frequency,- which consists intransmitting the bands inboth directions in normal relatin over a portion of 'the transmissioncircuit, inverting the order in the spectrum of the bands transmittingin one direction, inverting the order of the bands transmittingintheopposite direction,

.A inverting the. order of the individual frequencies in each band,transposing the two 5 sets of bands transmitting in oppositedirectionsso that each set occupies substantially 'the frequency spacenormally allotted to thev other, transmitting the bands thus invertedand transposed over another portion of the 0 transmission circuit,andthen reinverting and retransposing the bands so that they will assumetheir original positions. l

4. In a carrier system comprising a plurality of frequency bandsnormally arranged in a given order inthe frequency spectrum fortransmitting in one direction, a corresponding number of bands alsonormally arranged in-a, given 'order for transmitting inthe op positedirection, the method of equalization `with respect to frequency whichconsists in transmitting the bands in both directions in normal relationover a portion of the transmission circ-uit, then beating thefrequencies of all the bands transmitting in'botli direc- `$5 tions witha frequency substantially equal to two sets of bands transmitting inopposite' `directions will bev transposed.'

5. vA carrier system `comprising a plurality of frequency bandsnormallyarranged in a Lgiven order in the frequency' spectrum fortransmitting in one direction, a corresponding number of bands alsoarranged in a given order for transmitting in the opposite direction,means for transmitting the bands of both groups in normal relation overa portion of a transmission circuit, means comprisingv repeaterarrangements for inverting the order in the frequencyspectrumof e bandstransmitting'in one direction, inverting the order of the bands-transmitting in the opposite direction, at the saine time inverting theorder ofthe invidiial frequencies in each band, and for ,transposingthetwo sets of bands transnumber of bands also normally ar-' mitting inoppositedirectionsso t each set occupies substantially the frequencyspace normally allotted to theother, and means to transmit the bandsthus inverted and transposed over another portion of the transmissioncircuit.

6. A carrier system comprising a plurality of frequency bands normallyarranged ina giyen order in the frequency spectrum for 'transmitting inone direction, a corresponding number of bands alsoirranged in a givenorder for transmittinggrn the oppositedirection, means for transmittingthe bands of both groups in normal relation over a porv tion of atransmission circuit, means comprising repeater arrangements forinverting the order in the frequency spectrum of the bands transmittingin one direction, inverting the vorder of the bands transmitting in theopposite direction, and for transposing the two sets of the bandstransmitting in opposite directions so that each set occupiessubstantially the frequency space vnormally allotted to the other, andmeans for transmitting the .bands thus inverted and transposed overanother portion of the transmission circuit.

7. A carrier system comprising a plurality of frequency bands normallyarranged in a g'iven order in the frequency spectrum for transmitting inone/direction, a corresponding number of bands a'lso arranged in a givenrorder for transmitting in the opposite direction, means fortransmitting the bands of both groups inA normal relation over a portiono f a transmission circuit, means comprising repeater arrangements forinvert-ing` the order in the frequency Ispectrum of the bandstransmitting in one direction, inverting the order. of the bandstransmitting in the i opposite direction, at the same time inverting theorder of the individual frequencies in each band, and for transposingthe `two setsof bands transmitting in opposite directions so that eachset occupies substantially the frequency space normally 4allotted to'the other, means to transmit the bands thus inverted and transposed overanother portion of the transmission circuit, and meansto reinvert andre-transpose the bands so that they will assume their originalpositions.

8. A carrier system comprising a plurality` ,of frequency bands normallyarranged in a given order in the frequency spectrum for transmitting inone direction, a corresponding number of bands also arranged in a givenordei' for transmitting in the opposite direction, meansforftransmitting the bands ofl both groups in normal relation over aportion of a transmission circuit, means to beat the frequencies of allo'f the 'banyds transmitting in both directionswith a frequencysubstanlowest frequency of the lowest-band involved,

means to select the difference frequencies re- 'tially equal to the sumof the highest frequency in the highest band involved and the sultingfrom the modulation A of each band by said beating frequencies, means toapply the selected frequencies to another portion of the transmissioncircuit whereby, during trans-l mission over said portion of thecircuit, the frequencies in each individual band will be in invertedorder, the bands of, each set trans mitting in a given direction will bein ini verted order, and the two-sets of bands transmittiilg in'oppositedirections will be transposed;` 1

9. Ina multi-channel carrier signaling sysv 1 tem, the method ofequalizing all channel bands with respect to frequency, which con- 15sists in transposing at a single point the fre quency locationwith'resp'ect to each other of all of the channel 4bands transmitted in'both directions overfthe high frequency line circuit. 10; Inl amulti-channel carrier signaling l system, the method of. equalizing allchannel bands withv respect to frequency, which con sists in transposingat each of several repeater points the frequency location withrespect'to each other 'of all ofthe channel bands trans- I I ',mitted inbothdirections over the 4high frequency line circuit.

11. In a multi-channel carrier signaling system means for transposingthe frequency 30 location of alllsignal bands transmitted over thehigh-fr`equency circuit comprising filters to separate thevbandstransmitted in opposite directions and a modulator for changing the"position ofeach group of 'bands transmitted in a single direction andfor changing the relative position of the bands within Aeach group.y

In testimony whereof, have signed myname to thisspecilicat-ion this 21stday'of ifdoctober, 1927.

ESTILL 1 GREEN.

